Today's integrated circuits include a vast number of devices. Smaller devices and shrinking ground rules are the key to enhance performance and to improve reliability. As FET (Field Effect Transistor) devices are being scaled down, the technology becomes more complex, and changes in device structures and new fabrication methods are needed to maintain the expected performance enhancement from one generation of devices to the next. The mainstay material of microelectronics is silicon (Si), or more broadly, Si based materials. One such Si based material of importance for microelectronics is the silicon-germanium (SiGe) alloy.
There is great difficulty in maintaining performance improvements in devices of deeply submicron generations. Various detrimental device effects become more serious with successively decreasing device dimensions. For instance, with shortening gate lengths the so called short channel effects, most notably the “drain induced barrier lowering” pose severe roadblocks to miniaturization. Also, parasitic capacitances, such as source and drain capacitance, stand in the way of device performance. Several avenues are being explored in the art for keeping device performance improvements on track. One approach to deal with unwanted device capacitance is to utilize so called semiconductor on insulator (SOI), usually Si on insulator, technology. SOI devices typically are fabricated in a thin semiconductor layer disposed over an insulator layer. Most commonly, the insulator layer is a so called buried oxide layer on a Si substrate.
In general, FET devices built in SOI technology have the advantage of a lowered source/drain capacitance in comparison to devices built on bulk substrates. When SOI field effect device dimensions are being reduced, typically the SOI Si layer thickness is also reduced in order to have better short channel control of the threshold voltage (Vt). Eventually, the reduction of the SOI Si layer thickness results in FETs with floating body. A floating device body means that the device body potential is not tied to an applied voltage through a body contact. In such a case the threshold voltage, Vt, control of the device is more difficult. Field effect devices built on a bulk Si substrate can have body contact, but suffer from higher source/drain capacitance. It would be desirable to have a device which combines lowered source/drain capacitance, with the capacity for body contact.